1. Field of the Invention
The present invention relates to phosphorous steel powder mixtures to be used within the powder metallurgy. In addition to iron an phosphorus these powder mixtures can contain other alloying elements common within this technique, such as copper, nickel, molybdenum, chromium and carbon.
2. DESCRIPTION OF THE PRIOR ART
The use of phosphorus as an alloying element within the powder metallurgy has been known since the forties. Sintered steel alloyed with phosphorus has substantially improved strength characteristics in relation to non-alloyed sintered steel. Already at an early date there were for this object used mixtures of pure iron powder and ferrophosphorus powder. However, the ferrophosphorus first used has a composition which made it extremely hard and caused a considerable wearing of the tools. This drawback has been reduced to an acceptable degree by using a ferrophosphorus powder having a lower content of phosphorus and thereby reduced hardness, see for example Swedish Patent. No. 372,293.
However, sintered details manufactured by pressing and sintering such steel powder mixtures sometimes have an unacceptable brittleness. This is revealed for example by the fact that a population of sintered tests bars made from these mixtures can comprise individuals having extremely reduced mechanical characteristics especially with regard to impact strength and permanent strain after rupture (break elongation). As the advantage of phosphorus alloyed sintered steels is high strength in combination with very good strain characteristics the above brittleness risks are very serious.
Said brittleness risk has shown up to be present when the ferrophosphorus is of such composition that there is established a liquid phase at the sintering temperature. At the usually used sintering temperatures, 1040.degree. C and above that, this fact provides that phosphorus contents of more than 2.8% in the ferrophosphorus give a sintered material having an increased brittleness risk. The fact that ferrophosphorus having a high phosphorus content is used in spite of this drawback is dependent on the favorable sintering process which is provided by the liquid phase and the favorable distribution of the phosphorus in turn providing for a rapid indiffusion thereof which is obtained because of the fact that the ferrophosphorus provides for a liquid phase. cl SUMMARY OF THE INVENTION
Thus, the object of the present invention is to solve said problems with regard to the brittleness of sintered steel manufactured from a mixture of iron powder and a ferrophosphorus powder having a phosphorus content exceeding 2.8%. The solution of the problem has proved to reside in the use of a ferrophosphorus powder having a low content of impurities, especially impurities sensitive to oxidation. A further improvement can be obtained if the ferrophosphorus powder also has a small maximum particle size.
A phosphorus steel powder according to the invention for manufacturing sintered details having an extremely small tendency to brittleness ruptures consists of iron or steel powder substantially free from phosphorus, mixed with a phosphorus powder containing in all less than 4%, preferably less than 3% impurities which are at the sintering temperature more easily oxidized than the main components iron and phosphorus. Furthermore, the particles of the ferrophosphorus powder shall have a maximum size of 20 .mu.m, preferably a maximum size of 10 .mu.m. The phosphorus content of the ferrophosphorus powder shall exceed 2.8% and in order to reduce the wearing of the tools the phosphorus content shall be less than 17%. If the ferrophosphorus powder is manufactured by grinding piece goods the phosphorus content shall exceed 12% and shall preferably be between 14 and 16%. The phosphorus content of the preferred mixture is between 0.2 and 1.5%.
In this case there is a great difference between the particle sizes of the powder components in the mixture leading to an especially great risk of segregation and thereby of a discontinuous distribution of the alloying elements. In order to reduce the tendency of the mixture to segregate after the mixing operation 50 - 200 g of a light material oil per metric ton powder can be added during the mixing operation. Thereby the fine alloying particles are brought to adhere to the coarser iron powder particles.
In order to remove the protection against segregation the iron-ferrophosphorus mixture is heated with or without the addition of oil in reducing atmosphere to a temperature of between 650.degree. and 900.degree. C for a period of 15 min. to 2 hours. Thereby, the powder is loosely sintered together so that a following cautious disintegration has to be carried out in order to restore the original particle size. The powder provided in this way has iron particles with particles of the fine grained ferrophosphorus powder sintered thereto.
The methods described above in order to avoid segregation can be performed to a mixture having an increased content of the phosphorous powder. The concentrate so obtained can be mixed with the iron powder to provide for the desired phosphorus content in the final product.